1. Field of the Invention
The present invention relates to a local oscillating circuit suitable for a television tuner.
2. Description of the Related Art
Referring to FIG. 2, a conventional local oscillating circuit used in a television tuner will be described. First, an integrated circuit 60 includes a pair of oscillating transistors 61 and 62 whose emitters are connected to each other, and a constant current source 63. Also, a voltage is applied to each of the collectors of the pair of oscillating transistors 61 and 62 through a power-feeding resistor. Further, the integrated circuit 60 includes first to fourth coupling capacitors 64 to 67.
The integrated circuit 60 is provided with a first terminal 60a and a second terminal 60b. To the first terminal 60a, a base of the first oscillating transistor 61 is connected through the first coupling capacitor 64 and the collector of the second oscillating transistor 62 is connected through the second coupling capacitor 65. Also, to the second terminal 60b, the collector of the first oscillating transistor 61 is connected through the third coupling capacitor 66 and a base of the second oscillating transistor 62 is connected through the fourth coupling capacitor 67. In addition, the integrated circuit 60 includes a diode 68 whose cathode is connected to the first terminal 60a and anode is connected to a ground, and a diode 69 whose cathode is connected to the second terminal 60b and anode is connected to a ground.
Further, the integrated circuit 60 includes a pair of oscillating transistors 71 and 72 whose emitters are connected to each other, and a constant current source 73. Also, a voltage is applied to each of collectors of the pair of oscillating transistors 71 and 72 through a power-feeding resistor. Further, the integrated circuit 60 includes a fifth coupling capacitor 74 connected to a base of the third oscillating transistor 71, a sixth coupling capacitor 75 connected to the collector of the fourth oscillating transistor 72, and a capacitor 76 for connecting a base of the fourth oscillating transistor 72 to a ground.
Furthermore, the integrated circuit 60 is provided with a third terminal 60c, the fifth coupling capacitor 74 is connected between the third terminal 60c and the base of the third oscillating transistor 71, and the sixth coupling capacitor 75 is connected between the third terminal 60c and the collector of the fourth oscillating transistor 72. In addition, the integrated circuit 60 includes a diode 77 whose cathode is connected to the third terminal 60c and anode is connected to a ground. The above-mentioned three diodes 68, 69, and 77 serve as protection diodes against electrostatic damage.
Outside the integrated circuit 60 are provided a first resonating circuit 81 and a second resonating circuit 82. The first resonating circuit 81 is a parallel resonating circuit composed of an inductance element 81a and a varactor diode 81b and is connected between the first terminal 60a and the second terminal 60b. An anode of the varactor diode 81b is DC connected to a ground through a resistor 81c, and a tuning voltage Vt is applied to a cathode of the varactor diode 81b. Such a local oscillating circuit is used as a balanced local oscillator for receiving high-band television signals within a VHF band.
In addition, the second resonating circuit 82 is a parallel resonating circuit composed of an inductance element 82a and a varactor diode 82b, and one end of the second resonating circuit 82 is connected to the third terminal 60c and the other end thereof is connected to a ground. Also, an anode of the varactor diode 82b is DC connected to a ground through the inductance element 82a, and the tuning voltage Vt is applied to a cathode thereof. Such a local oscillating circuit is used as a non-balanced local oscillator for receiving low-band television signals (for example, see Japanese Unexamined Patent Application Publication No. 2003-179430 (FIG. 3)).
However, since a local oscillating circuit for receiving high-band signals and a local oscillating circuit for receiving low-band signals are provided independently in the configuration described above, the configuration is complex although a sufficient oscillating frequency band can be obtained in each receiving band.